Noble metal nanoparticles, a process for preparing these and their use

ABSTRACT

Nanoparticles which contain noble metals alone or noble metals in combination with base metals. The nanoparticles are embedded in an aqueous solution of a temporary stabilizer based on a polysaccharide.

This application is a divisional of U.S. patent application Ser. No.09/910,959, filed Jul. 24, 2001, which is herein incorporated byreference.

INTRODUCTION AND BACKGROUND

The present invention provides noble metal-containing nanoparticles forproducing membrane electrode assemblies (MEAs) for fuel cells, inparticular for low temperature fuel cells, for example polymerelectrolyte membrane fuel cells (PEM fuel cells) and direct methanolfuel cells (DMFC). New types of colloidal solutions which contain thenoble metal alone or in association with other metals are described,wherein the metals are in the form of nanoparticles embedded in atemporary stabilizer. The nanoparticles are used to produceelectrocatalysts and catalysed components for fuel cells. Using thesenanoparticles, catalyzed ionomer membranes, catalyzed gas diffusionelectrodes (so-called “backings”) and membrane electrode assemblies canbe produced.

Fuel cells convert a fuel and an oxidizing agent which are spatiallyseparated from each other at two electrodes into electricity, heat andwater. Hydrogen or a hydrogen-rich gas may be used as the fuel, andoxygen or air as the oxidizing agent. The process of energy conversionin the fuel cell is characterized by a particularly high efficiency. Forthis reason, fuel cells in combination with electric motors are becomingmore and more important as an alternative to traditional internalcombustion engines. The PEM fuel cell is suitable for use as an energyconverter in motor vehicles because of its compact structure, its powerdensity and its high efficiency.

The PEM fuel cell consists of a stacked arrangement (“stack”) ofmembrane electrode assemblies (MEAs), between which are arranged bipolarplates for supplying gas and conducting electricity. A membraneelectrode assembly consists of a solid polymer electrolyte membrane,both sides of which are provided with reaction layers which containcatalysts. One of the reaction layers is designed as an anode for theoxidation of hydrogen and the second reaction layer is designed as acathode for the reduction of oxygen. On these reaction layers aremounted so-called gas distributor structures or gas diffusion layersmade of carbon fibre paper, carbon fibre woven fabric or carbon fleece,which facilitate good access by the reaction gases to the electrodes andeffective removal of the cell current. The anode and cathode containso-called electrocatalysts which catalytically support the particularreaction (oxidation of hydrogen at the anode and reduction of oxygen atthe cathode). Metals from the platinum group in the periodic system ofelements are preferably used as the catalytically active components. Inthe majority of cases, so-called supported catalysts, in which thecatalytically active platinum group metal has been applied in highlydispersed form to the surface of a conductive support material, areused.

The polymer electrolyte membrane consists of proton-conducting polymermaterials. These materials are also called ionomers for short in thefollowing. A tetrafluorethylene/fluorovinylether copolymer with acidfunctions, in particular sulfonic acid groups, is preferably used. Suchmaterials are sold, for example, under the tradenames Nafion® (E.I.DuPont) or Flemion® (Asahi Glass Co.). However, other, in particularfluorine-free, ionomer materials such as sulfonated polyetherketones orpolyarylketones or polybenzimidazoles but also ceramic materials can beused.

The performance data for a fuel cell depends critically on the qualityof the catalyst layers applied to the polymer electrolyte membrane.These layers usually consist of an ionomer and a finely dividedelectrocatalyst dispersed therein. Together with the polymer electrolytemembrane, so-called three-phase interfaces are formed in these layers,wherein the ionomer is in direct contact with the electrocatalyst andthe gases (hydrogen at the anode, air at the cathode) introduced to thecatalyst particles via the pore system.

To prepare the catalyst layers, ionomer, electrocatalyst and optionallyother additives are generally blended to form an ink or a paste. Toproduce the catalyst layer, the ink is applied by brushing, rolling,spraying, doctor blading or printing either to the gas diffusion layer(e.g. carbon fleece or carbon fibre paper) or directly to the polymermembrane, dried and optionally subjected to a secondary treatment. Inthe case of coating the ionomer membrane with a catalyst layer, thenon-catalyzed gas diffusion layers are then mounted on the membrane onthe anode and cathode faces and a membrane electrode assembly is thenobtained. Alternatively, the catalyst layers may also be applied to thegas diffusion layers. These gas diffusion electrodes (gas diffusionlayers plus catalyst layers) are then laid on the two faces of theionomer membrane and laminated with this, wherein a membrane electrodeassembly is also obtained. The prior art in this area is described inpatent documents U.S. Pat. Nos. 5,861,222, 5,211,984 and 4,876,115.

The present invention provides noble metal-containing nanoparticleswhich can be used for the production of catalyzed components andmembrane electrode assemblies for low temperature fuel cells (PEMFC,DMFC, AFC or PAFC). The object of the invention are new types ofpreparations, or colloidal solutions, of noble metal-containingnanoparticles which are embedded in a suitable temporary stabilizer.

Colloidal nanoparticle solutions have been known for a long time. Forexample, they are used to produce noble metal supported catalysts. Thus,U.S. Pat. No. 3,992,512 describes a process in which colloidal platinumoxide nanoparticles are prepared by decomposing platinum sulfite acid,fixing these to a supporting carbon black and then reducing to platinum.The process is complicated and expensive and provides onlyelectrocatalysts which are contaminated with sulfur due to usingsulfur-containing precursor compounds for the platinum. Stabilizers arenot used.

DE 197 54 304 A1 describes platinum-containing nanoparticles which areembedded in a polymeric betaine. Polymeric carbobetaine, phosphobetaineand sulfobetaine, which are built up from a non-branched polymethylenemain chain and side chains with different types of betaine groups havingdegrees of polymerization of 50 to 10,000, are described. The method fordecomposing these stabilizers is not described. It has been shown thatthese stabilizers adhere firmly to the noble metal surface, due to theirlong polymethylene main chains, and thus contaminate the catalyticallyactive catalyst surfaces. For this reason, these nanoparticles are notvery suitable as catalytically active species for membrane electrodeassemblies in fuel cells. Nothing is reported about the furtherprocessing of these in order to produce catalyzed systems (catalysedionomer membranes, gas diffusion electrodes, etc.).

Furthermore, DE 44 43 705 A1 discloses noble metal colloids which arestabilized with surfactants (such as, for example, fatty alcoholpolyglycol ethers or amphiphilic betaines) and can be used for preparingsupported electrocatalysts. After attaching these noble metal colloidsto the support material, aftertreatment is required in order to removethe surfactants used for stabilizing purposes. During thisaftertreatment (generally thermal pyrolysis at temperatures above 400°C.) the colloid particles sinter so that coarse crystallites areproduced.

Furthermore, DE 197 45 904 A1 describes a polymer-stabilized metalcolloid solution which contains a cation exchange polymer forstabilizing purposes. Here, the noble metal nanoparticles areprecipitated in the presence of an ionomer solution (e.g. Nafion®) andisolated as a dry powder. Investigations by the inventors of the presentinvention have shown that this process does not lead to stable liquidcolloid preparations because the ionomer has no surfactant propertiesand in addition is itself present as particles in the size range 5 to 20nm (see also X. Cheng et al., J. Power Sources 79 (1999) 75-81). Inaddition, our work has shown that this process has considerabledisadvantages because it provides nanoparticles which are heavilycontaminated with foreign ions such as, for example, chloride or sodium.The presence of chloride in particular leads to corrosion and reducedresistance to ageing of the catalyst components prepared using thismetal colloid preparation.

Therefore it is the object of the present invention to provide noblemetal-containing nanoparticles which form stable solutions over a longtime due to the use of a suitable, temporary, stabilizer and containonly marginal amounts of impurities (halogen ions, alkali metal ions,borate, etc.), which are insignificant for use in fuel cells. They areintended to be used directly for catalyzing ionomer membranes and gasdiffusion layers for PEM fuel cells, which means that the temporarystabilizer (or protective colloid) has to be completely removable bymeans of a gentle process without damaging the polymer electrolytemembrane. Furthermore, the nanoparticles are intended to be capable ofbeing prepared in aqueous medium without the addition of organicsolvents.

SUMMARY OF THE INVENTION

This object is achieved, according to the invention, by nanoparticleswhich contain the noble metals only or noble metals in combination withbase metals and are characterised in that they are embedded in anaqueous solution of a temporary stabilizer based on a polysaccharide.

According to the invention, polysaccharides are used to stabilize thenanoparticles. Suitable polysaccharides are described in UllmannsEnzyklopädie der technischen Chemie, 4th edition, vol. 19, p. 233 etseq. Polysaccharides are water soluble, highly polymer carbohydratecompounds, which consist of monosaccharide units linked together by aso-called glycosidic bond. When forming this bond, the anomeric hydroxylgroup of the monosaccharide reversibly condenses with the hydroxy groupof another monosaccharide to form a disaccharide, oligo- or finally apolysaccharide molecule. A single polysaccharide molecule can contain upto several ten thousands of various monosaccharide units.Polysaccharides, which are composed of various types of monosaccharideunits are called heteropolysaccharides, those which contain only onemonosaccharide type are called homopolysaccharides. The polysaccharidesdiffer in their molecular weight, their composition and, most important,in their water solubility.

It was discovered that the polysaccharides suitable for use in thepresent invention must be highly water soluble. Most commonpolysaccharides and gums cannot be dissolved in water at concentrationshigher than about 5 wt. % because of their very high viscosities andtheir gelling behaviour. The preferred polysaccharides exhibit a watersolubility of about 5 to 40 wt. % while still maintaining a lowviscosity solution. The preferred polysaccharides areheteropolysaccharides such as gum arabic, xanthan gum, tragacanth gum ormixtures thereof.

Furthermore it was found that the water based solution of thepolysaccharide has to be preferably in the pH neutral form. A pH rangeof 5 to 8, preferably 5.5 to 7.5 and most preferably 6 to 7 is requiredto ensure stability of the polysaccharide. At a lower, more acidic pH,as well as at higher alkaline pH values, the glycosidic bonds of thepolysaccharide are broken up and the macromolecule is destroyed. Thiseffect is used in turn to remove traces of the stabilizer after thecolloidal noble metal particles have been deposited on the suitablesubstrate material (ionomer membranes, gas diffusion electrodes orcarbon black supports) as described further below in this invention.

When properly selected, the stabilizers mentioned are able to keep thecolloidal preparation of nanoparticles, even in high concentrations,stable for a long time. For this purpose, it has proven advantageous toadjust the ratio by weight of nanoparticles to stabilisers to a valuebetween 10:1 and 1:10, preferably between 5:1 and 1:5.

The temporary stabilizers used have to be capable of being removedeffectively. Particularly important here is an easy decomposition (i.e.breaking down of the main chain in the polymer into low molecular weightfragments). During the course of trials, it has been shown that thepolysaccharides are extremely suitable as temporary stabilisers. Asdescribed previously, in these compounds, the glycosidic bonds betweenthe individual monosaccharides or sugar monomers break readily whentreated with acids or alkalis. They depolymerize and break down into lowmolecular weight constituents. This decomposition process also takesplace during pyrolysis at temperatures up to 250° C. The low molecularweight fragments can be readily removed, for example by washing out.

Nanoparticles according to the invention may contain one or more noblemetals and optionally in addition at least one base metal. Nanoparticlesaccording to the invention preferably contain at least one noble metalfrom the group platinum, palladium, rhodium, iridium, ruthenium, osmium,gold and silver. Suitable base metals are iron, cobalt, nickel, copper,titanium, vanadium, chromium, manganese, molybdenum, tungsten andrhenium. The particle size of the nanoparticles is between 0.1 and 100,preferably between 1 and 20 and in particular between 1 and 5 nm. Theparticle size can be determined by means of transmission electronmicroscopy (TEM).

The concentration of noble metal nanoparticles in the aqueous colloidsolution is 0.01 to 500 g/l (0.001 to 50 wt. %), typically 0.1 to 200g/l (0.01 to 20 wt. %).

Nanoparticles according to the invention can be obtained by reducingprecursor compounds of the desired noble metals and optionally basemetals with a total chlorine concentration of less than 500 ppm in anaqueous solution in the presence of the stabiliser, using a reducingagent.

Suitable reducing agents for the preparation according to the inventionare those which decompose to produce no residues or which leave behindno problematic ionic or organic impurities during the reduction process.Examples of these are hydrogen, hydrazine, formaldehyde, or else loweraliphatic alcohols such as ethanol or isopropanol which decompose togive gaseous constituents due to the reduction reaction. The reducingagent is added directly to the reaction solution, with stirring, whereintemperatures of up to 95° C. are optionally used. After completion ofthe reduction the colloidal solution of the nanoparticles in principledoes not contain any more reducing agents. Surplus reducing agents aredestroyed due to treatment at elevated temperatures of up to 95° C.

The following halogen-free, or low-halogen, compounds are used, forexample, as noble metal precursor compounds for preparing thenanoparticles:

-   for Pt: hexahydroxoplatinic(IV) acid, ethylammonium    hexahydroxoplatinate, tetraammineplatinum(II) nitrate, platinum(IV)    nitrate, tetraammineplatinum(II) hydroxide solution-   for Pd: tetraamminepalladium(II) nitrate, palladium(II) nitrate,    palladium(II) sulfate hydrate-   for Ru: trinitratonitrosylruthenium(II), ruthenium(III) oxalate    hydrate etc.-   for Rh: rhodium(III) nitrate hydrate, rhodium(III) sulfate solution    etc.

Corresponding compounds may also be used for the noble metals Au, Ag, Irand Os. The precursor compounds used for the base metals mentioned arechlorine-free salts of the base metals, preferably nitrate compounds.

In general, the total chlorine content of the precursor compounds usedshould be less than 500 ppm. Determination of the total chlorine contentincludes both the free and also the bonded chlorine and is performed,for example, by ion chromatography (IC), in aqueous solution, afterworking up the substance in a suitable manner.

The total chlorine content of the noble metal solution according to theinvention is typically less than 100 ppm, preferably less than 50 ppm.

Nanoparticles according to the invention may be used to preparesupported electrocatalysts. A particular advantage of the nanoparticles,however, is that they can also be used directly, that means without asupport, to prepare catalyst layers for ionomer membranes and gasdiffusion layers and also to impregnate the ionomer membranesthemselves.

In the following, some of these types of use are described in moredetail.

For the preparation of supported electrocatalysts, the noble metalnanoparticles are deposited onto a suitable carbon black material.Several methods (for example impregnation, soaking or incipient wetnesstype methods and others) can be used for this process. Hereby,electrocatalysts exhibiting a very high noble metal dispersion (i.e.noble metal surface area) are obtained, even at very high noble metalloading of the carbon black support. Investigations of the presentinventors have shown, that electrocatalysts with a noble metal loadingof up to 80 wt. % on carbon black can be prepared with particle sizes inthe range of 2 to 5 nanometer. After depositing the nanoparticles on thecarbon black support, the stabilizer can be removed under mildconditions, that is with acidic or alkaline hydrolysis or by thermaldecomposition at temperatures of up to 250° C.

Direct use of the nanoparticles for catalysing the various components ina fuel cell is enabled in that the protective colloid, or temporarystabilizer, decomposes under relatively mild conditions and can bewashed out so that damage to the components in the fuel cell does notoccur. This produces a considerable simplification in and reduction incosts of the production process for membrane electrode assemblies. Inaddition, the process has the advantage that the high surface area anddispersion of the nanoparticles is retained and is not distorted by hightemperature tempering processes. This leads to very good performance bythe membrane electrode assemblies prepared in this way so that theplatinum loading can be kept low.

In the case of coating an ionomer membrane, the preparation with thenanoparticles, optionally mixed with other additives such as, forexample, dissolved ionomer, carbon black or further electrocatalysts, isapplied to the membrane in a spray process, by brushing or immersing orby means of screen printing. After coating, the temporary stabilizer isdecomposed by treating with acid or alkali and it is then washed out.Dissolved ionomer is obtainable in aqueous solution with low molecularweight aliphatic alcohols (Fluka, Buchs; Aldrich, Steinheim). Aqueoussolutions of the ionomer in higher concentrations (10 wt. % and 20 wt.%) can be prepared therefrom.

Ionomer membranes and also the ionomer contained in the catalyst layerscan be used in an acidic proton-conducting H⁺ form or, after exchangingthe protons for monovalent ions such as, for example, Na⁺ and K⁺, in anon-acidic Na⁺ or K⁺ form for preparing membrane electrode assemblies.The non-acidic form of polymer membranes is usually more stable towardsthermal stress than the acidic form and is therefore preferably used.Before using the membrane electrode assembly, however, the polymerelectrolyte has first to be returned to its acidic, proton-conductingform. This is achieved by so-called reprotonation. Reprotonation isperformed by treating the membrane electrode assembly in sulfuric acid.

Reprotonation with sulfuric acid can therefore be combined in a simplemanner with decomposition of the temporary stabilizer. This simplifiesthe production process for membrane electrode assemblies. The ionomermembrane catalyzed in this way is then completed with 2 gas diffusionelectrodes to give a 5-layered membrane electrode assembly.

As an alternative to coating the ionomer membrane, the gas diffusionlayers may also be coated with the catalytically active component. Forthis purpose, the colloidal preparation of nanoparticles, optionallywith the additives mentioned above, is applied to a gas diffusion layer(gas distribution structure or “backing” consisting of carbon fibrepaper) using an appropriate method. The stabilizer is then removed by atempering process at temperatures below 250° C. and the catalysedelectrode, as the anode and cathode, are further laminated with anionomer membrane to give a 5-layered membrane electrode assembly.

Furthermore, the colloidal noble metal nanoparticles may also beprocessed to give a catalyst ink. Suitable catalyst inks are described,for example, in patent specification U.S. Pat. No. 5,861,222 in the nameof the applicant, wherein the supported catalysts used there may bereplaced entirely or partly by noble metal nanoparticles according tothe invention.

The colloidal solution of noble metal nanoparticles is also suitable forprecatalyzing ionomer membranes by impregnating the ionomer membrane inthe solution. In further steps, a catalyst ink is then applied to theprecatalyzed ionomer membrane, as described in the US patentspecification mentioned above. However, the precatalyzed ionomermembrane may also be assembled, together with catalysed gas diffusionelectrodes on the cathode and anode faces, and laminated to produce a5-layered membrane electrode assembly.

In a further type of use, a thin layer of the colloidal noble metalparticles according to the invention is also applied to a catalysed gasdiffusion electrode, for example by spraying or brushing. Themulti-catalyzed gas diffusion electrodes are then combined with theionomer membrane in a sandwich structure and optionally laminated.

In addition, combinations of the types of use described above arepossible. All these methods for catalysing, due to the use of thecolloidal noble metal particles according to the invention, lead to highcatalytic activity and electrical performance in the membrane electrodeassembly, and in the PEM fuel cell.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be further understood with reference to theaccompanying drawing, wherein:

FIG. 1 is a schematic cross section of a polymer electrolyte membranedirectly catalyzed with nanoparticles according to the invention;

FIG. 2 is a schematic cross section of an electrode backing directlycatalyzed with nanoparticles according to the invention;

FIG. 3 is a schematic cross section of an membrane electrode assembly(MEA) with catalyst layers comprising a supported electrocatalyst andnanoparticles according to the invention; and

FIG. 4 is a schematic cross section of a 5-layered membrane electrodeassembly according to the invention.

DETAILED DESCRIPTION OF INVENTION

Directly catalyzing the various components of a PEM fuel cell with noblemetal nanoparticles is accomplished by applying the aqueous solution ofthe stabilized nanoparticles with no further additives to the componentsin question by a spray process, by brushing or immersing or by means ofscreen printing. After coating, the temporary stabilizer is decomposedby treating with acid or alkali (depending on the type of stabilizer)and is then washed out.

FIG. 1 visualises such a coating on the opposing surfaces of a polymerelectrolyte membrane (1). The noble metal nanoparticles (2) are directlyapplied to the surfaces of the ionomer by a process as described above.

FIG. 2 shows a similar coating as in FIG. 1 on an electrode backingconsisting of a hydrophobic gas diffusion layer (3) with a carbon blackmicro layer (4) on one of its surfaces. A micro layer consists of amixture of a hydrophobic polymer and carbon black. The micro layer has amicroporosity and serves as an intermediate layer between the gasdiffusion layer and the catalyst layer of a MEA to improve theelectronic connection between both. In FIG. 2 the noble metalnanoparticles (2) are directly deposited onto the micro layer. Since anelectrode backing can withstand much higher temperatures than thepolymer membrane (340° C. instead of only 150° C.) the temporarystabilizer can be decomposed in this case thermally by heating thecoated electrode backing up to a temperature of 250° C.

FIG. 3 shows the structure of a polymer electrolyte membrane (1) coatedwith two catalyst layers (5) and (6). The catalyst layers comprise asupported electrocatalyst (7) and unsupported nanoparticles (8). Thesupported electrocatalyst and the unsupported nanoparticles are bothdispersed in a matrix of a ionomer (9). The catalyst layers (5) and (6)may be the same or different. In the final fuel cell one of thesecatalyst layers functions as the anode and the other as the cathode ofthe fuel cell.

FIG. 4 visualizes a polymer electrolyte fuel cell comprising themembrane electrode assembly of FIG. 3 complemented by two electrodebackings consisting of a hydrophobic gas diffusion layer (3) and a microlayer (4). Taking the electrode backings as one layer, the structure ofFIG. 4 can be viewed as a 5-layered membrane electrode assembly.

The invention is explained in more detail in the following by the use ofa few examples. In the examples, membrane electrode assemblies wereprepared by using nanoparticles according to the invention and theirelectrochemical performance data were characterized.

For this purpose, the membrane electrode assemblies were processed togive PEM single cells and their characteristics (change involtage/current density plot) were measured at a pressure of about 1 bar(abs.) when operated with hydrogen/air or with reformate/air. The sizeof each cell was 50 cm², the cell temperature was 75° C. From thecharacteristic plots, the cell voltage obtained at a current density of500 mA/cm² was recorded as a measure for the electrocatalyticperformance of the cell.

EXAMPLE 1

a) Preparing Pt Nanoparticles

11.1 g of a solution of bis(ethanolammonium) hexahydroxoplatinate (Ptcontent 9 wt. %; total chlorine content <100 ppm; from dmc², Hanau) wereadded dropwise to 1.5 l of fully deionized water in which 1.0 g of gumarabic (Merck) had previously been dissolved. Then, 1 l of ethanol wasadded with stirring and the resulting mixture was heated, wherein themixture turned black. The solution was kept under reflux for one hour at85° C. and then concentrated to a volume of 100 ml by evaporation. Thecolloidal solution prepared in this way had a pH value of 5.9 andcontained 10 g Pt/l (1 wt. % Pt) and 10 g/l (1 wt. %) of the stabilizergum arabic. The ratio of Pt nanoparticles to stabilizer was thus 1:1.The total chlorine content of the solution was less than 10 ppm. Theaverage size of the Pt particles was determined using TEM (transmissionelectron microscopy) and was 2 nm.

b) Catalyzing Ionomer Membranes

5.6 g of the colloidal solution (Pt content 1 wt. %) were dispersed with0.4 g of an aqueous solution of Nafion (10 wt. % in water) and 0.1 g ofcarbon black (type: Vulcan XC-72, from Cabot) and the resulting ink wasapplied in a spray process to the front and rear faces of a Nafionmembrane (type: Nafion 112, thickness 50 μm, from DuPont). Then dryingwas performed at temperatures of 80° C. in a circulating air oven. Thetotal Pt loading on the front and rear faces of the membrane was 0.2 mgPt/cm². After drying, the catalyzed membrane was treated for 30 min in asulfuric acid bath (0.5 normal, pH=0.3) and then washed with water.After that, it was placed between two non-catalyzed gas diffusion layersand incorporated into a PEM single cell.

When operating with hydrogen/air (pressureless operation, about 1 bar),a cell voltage of 600 mV was produced with a current density of 500mA/cm².

c) Catalyzing Gas Diffusion Electrodes

0.4 g of an aqueous solution of Nafion (10% in water) were added to 5.6g of the colloidal solution (concentration: 1 wt. % Pt) and the mixturewas applied in a spray process to two gas diffusion layers (type:Standard ELAT, ETEK, Natick, USA) provided, in a known manner, with acarbon black micro layer. The Pt loading on the anode electrode was 0.1mg/cm², that on the cathode electrode was 0.15 mg/cm². Then drying wasperformed at temperatures of 80° C. in a circulating air oven and atempering process was performed under nitrogen at 250° C. The electrodesprepared in this way were combined with an non-catalyzed membrane togive a 5-layered membrane electrode assembly which had a total Ptloading of 0.25 mg Pt/cm². In a PEM single cell, very good performancevalues were obtained when operating with hydrogen/air (pressurelessoperation at about 1 bar; cell voltage: 600 mV with a current density of500 MA/cm²).

EXAMPLE 2

a) Preparation of Pt/Ru Nanoparticles

7.28 g of a solution of bis(ethanolammonium) hexahydroxoplatinate (Ptcontent 9 wt. %; total chlorine content <100 ppm; from dmc², Hanau) and2.265 g of a solution of ruthenium nitrosylnitrate (Ru content 15 wt. %,total chlorine content <200 ppm; from dmc², Hanau) were added dropwiseto 1.5 l of fully deionized water, in which 1.0 g of gum arabic (Merck)had been dissolved. Then 1 l of ethanol was added with stirring and theresulting mixture was heated, wherein it turned black. The solution washeld under reflux for one hour at 85° C. and then concentrated byevaporation to a volume of 100 ml. The colloidal solution obtained inthis way had a pH value of 5.7 and contained 10 g PtRu/l (1 wt. % PtRu,atomic ratio 1:1) and 10 g/l (1 wt. %) of the stabiliser gum arabic. Theratio of PtRu nanoparticles to stabilizer was thus 1:1. The totalchlorine content of the solution was less than 50 ppm. The average sizeof the PtRu particles was determined by TEM and was 2.5 nm.

B) Catalyzing an Ionomer Membrane

5.6 g of the colloidal solution (concentration: 1 wt. % PtRu) weredispersed with 0.4 g of an aqueous solution of Nafion (10% in water) and0.1 g of carbon black (type: Vulcan XC-72, from Cabot) and the resultingink was applied in a spray process to the anode face of a Nafionmembrane (type: Nafion 112, thickness 50 μm, from DuPont). Then dryingwas performed at temperatures of 80° C. in a circulating air oven. ThePt loading on the membrane on the anode face was 0.1 mg Pt/cm², the Ruloading was about 0.05 mg/cm². Then the cathode face of the ionomermembrane was catalyzed in the way described in example 1 (Pt loading 0.1mg/cm²). After drying, the complete membrane was treated in a sulfuricacid bath (0.5 normal, pH=0.3) for 30 min and then washed with water.After that the membrane coated with catalyst was placed between 2non-catalyzed gas diffusion layers and incorporated into a PEM singlecell. The total noble metal loading was 0.2 mg Pt/cm² and 0.05 mgRu/cm². The single cell test produced very good performance values whenoperating with reformate/air (reformate composition: 60 vol. % hydrogen,25 vol. % carbon dioxide, 15 vol. % nitrogen, 40 ppm CO, 2% air bleed,pressureless operation; cell voltage: 550 mV with a current density of500 mA/cm²).

EXAMPLE 3

2.22 g of a solution of bis(ethanolammonium) hexahydroxoplatinate (Ptcontent 9 wt. %; total chlorine content <100 ppm; from dmc², Hanau) wereadded dropwise to 1.5 l of fully deionized water in which 0.2 g ofKelzan (xanthan gum, Lubrizol-Langer, Bremen) had previously beendissolved. Then 1 l of isopropanol was added with stirring and theresulting mixture was heated, wherein it turned black. The solution washeld under reflux for one hour at 85° C. and then concentrated byevaporation to a volume of 100 ml. The colloidal solution obtained inthis way had a pH value of 5.6 and contained 2 g Pt/l (0.2 wt. % Pt) and2 μl (0.2 wt. %) of the stabilizer Kelzan. The ratio of Pt nanoparticlesto stabilizer was thus 1:1. The total chlorine content of the solutionwas less than 30 ppm. The average size of the Pt particles wasdetermined by TEM and was 2.5 nm.

An ionomer membrane was catalyzed in the same way as described inexample 1 and a membrane with a total platinum loading of 0.2 mg Pt/cm²was produced. In a PEM single cell, very good performance values wereobtained when operating with hydrogen/air (pressureless operation; cellvoltage: 630 mV with a current density of 500 mA/cm²).

EXAMPLE 4

2.2 g of a solution of bis(ethanolammonium) hexahydroxoplatinate (Ptcontent 9 wt. %; total chlorine content <100 ppm; from dmc², Hanau) wereadded dropwise to 1.5 l of fully deionized water in which 0.436 g of gumarabic (Merck, Darmstadt) and 0.137 g of chromium(III) nitratenonahydrate (total chlorine content <20 ppm, Merck) had previously beendissolved. The solution thus contained 0.2 g Pt (about 1 mmol) and 0.018g Cr (about 0.3 mmol) to prepare PtCr nanoparticles with a Pt:Cr-atomicratio of 3:1. Then 1 g of hydrazine hydrate (24% strength solution,Merck) was added dropwise with stirring and the resulting mixture washeated, wherein it turned black. The solution was held at boiling pointfor one hour and then concentrated by evaporation to a volume of 100 ml.The colloidal solution obtained in this way contained 2.18 g PtCr(3:1)/land 4.36 g/l of the stabilizer gum arabic. The ratio of PtCrnanoparticles to stabilizer was thus 1:2. The total chlorine content ofthe solution was less than 30 ppm. The average size of the PtCrparticles was determined by TEM and was about 3 nm.

An ionomer membrane was catalyzed in the way described in example 1.However, the cathode face of the membrane was coated with PtCr(3:1)nanoparticles and the anode face was coated with pure Pt nanoparticles.The membrane coated in this way had a total platinum loading of 0.2 mgPt/cm². Measurement in a PEM single cell when operating withhydrogen/air (pressureless operation, about 1 bar) provided very goodresults. The cell voltage was 720 mV with a current density of 500mA/cm².

EXAMPLE 5

Pt nanoparticles were prepared in the way described in example 1. Tocatalyse an ionomer membrane, the Pt nanoparticles were incorporatedinto a catalyst ink of the following composition:

15.0 g Pt supported catalyst (40 wt. % Pt on carbon black) 50.0 g Nafionsolution (10% in water) 30.0 g Pt nanoparticles (Pt content 1 wt. %) 5.0g Dipropylene glycol 100.0 g

The above catalyst ink contains a mixture of a conventional Pt supportedcatalyst and unsupported noble metal nanoparticles according to theinvention.

The ink was applied in a screen printing process to the anode andcathode faces of an ionomer membrane (Nafion 112) to give a membraneelectrode structure as shown in FIG. 3. The total Pt loading was 0.5mg/cm². Measurement in a PEM single cell operating with hydrogen/air(pressureless operation, about 1 bar) provided very good results. Thecell voltage was 710 mV with a current density of 500 mA/cm².

Further variations and modifications of the present invention will beapparent to those skilled in the art from the foregoing and are intendedto be encompassed by the claims appended hereto.

German priority application 100 37 071.3 of Jul. 29, 2000 is relied onand incorporated herein by reference.

1. A process for the catalytic coating of a polymer electrolyte membranewhich comprises the steps of: a) preparing an aqueous noble metalnanoparticle solution comprising noble metal nanoparticles embedded inan aqueous solution of a temporary stabilizer which is a polysaccharide,wherein said aqueous solution has a pH in the range of 5.5 to 7.5; b)applying said aqueous noble metal nanoparticle solution to the polymerelectrolyte membrane to form a coated polymer electrolyte membrane, andc) reprotonating the polymer electrolyte membrane and decomposing thetemporary stabilizer by treating the coated polymer electrolyte membranein sulfuric acid.
 2. The process according to claim 1 further comprisinga washing step.
 3. The process according to claim 1, wherein the aqueousnoble metal nanoparticle solution is applied to the polymer electrolytemembrane by a spray process, by a brushing process, by a immersingprocess, by an impregnating process or by a screen printing process. 4.The process according to claim 1, wherein the concentration of the noblemetal nanoparticles in the aqueous solution is in the range of 0.1 to200 g/l.
 5. The process according to claim 1, wherein the ratio of thenoble metal nanoparticles to the stabilizer is in the range of 5:1 to1:5 by weight.
 6. The process according to claim 1, wherein the noblemetal nanoparticles have a particle size in the range of 1 to 5 nm. 7.The process according to claim 1, wherein the aqueous noble metalnanoparticle solution further comprises an aqueous solution of dissolvedionomer.
 8. The process according to claim 1, wherein the aqueous noblemetal nanoparticle solution further comprises carbon black.
 9. Theprocess according to claim 1, wherein the polysaccharide is aheteropolysaccharide selected from the group consisting of gum arabic,xanthan gum, tragacanth gum and mixtures thereof.
 10. The processaccording to claim 1, wherein the aqueous noble metal nanoparticlesolution is obtained by reducing suitable precursor compounds of thenoble metal nanoparticles in the presence of the temporary stabilizerusing a reducing agent, which decompose during the reduction process.